U.S. patent number 5,266,361 [Application Number 07/674,370] was granted by the patent office on 1993-11-30 for preparation of a multilayer coating.
This patent grant is currently assigned to BASK Lacke + Farben Aktiengesellschaft DE/DE. Invention is credited to Ulrich Poth, Stephan Schwarte.
United States Patent |
5,266,361 |
Schwarte , et al. |
November 30, 1993 |
Preparation of a multilayer coating
Abstract
The invention relates to a process for the preparation of a
multicoat coating which uses an aqueous, clear topcoat composition
containing a polyacrylate resin as binder. The polyacrylate resin
can be obtained by adding an ethylenically unsaturated monomer (a1)
which contains carboxyl groups, and a mixture (b) consisting of
(meth)acrylic esters which are free from carboxyl groups and of
ethylenically unsaturated monomers which contain hydroxyl groups,
to an organic solvent or mixture of solvents either successively or
alternately in part amounts and polymerizing the mixture in the
presence of a radical-forming initiator, neutralizing the resultant
polyacrylate resin after termination of the polymerization and
dispersing it in water. The type and amount of (a1) and (b) are
chosen such that the polyacrylate resin has a hydroxyl value of 40
to 200, an acid value of 20 to 100 and a glass transition
temperature (T.sub.G) of -40.degree. C. to 60.degree. C.
Inventors: |
Schwarte; Stephan (Emsdetten,
DE), Poth; Ulrich (Munster, DE) |
Assignee: |
BASK Lacke + Farben
Aktiengesellschaft DE/DE (Munster, DE)
|
Family
ID: |
6364952 |
Appl.
No.: |
07/674,370 |
Filed: |
May 3, 1991 |
PCT
Filed: |
September 26, 1989 |
PCT No.: |
PCT/EP89/01126 |
371
Date: |
May 03, 1991 |
102(e)
Date: |
May 03, 1991 |
PCT
Pub. No.: |
WO90/03851 |
PCT
Pub. Date: |
April 19, 1990 |
Foreign Application Priority Data
|
|
|
|
|
Oct 12, 1988 [DE] |
|
|
3834738 |
|
Current U.S.
Class: |
427/407.1;
427/409; 524/512 |
Current CPC
Class: |
C09D
151/00 (20130101); C08F 265/02 (20130101) |
Current International
Class: |
C08F
265/02 (20060101); C08F 265/00 (20060101); C09D
151/00 (20060101); B05D 001/36 () |
Field of
Search: |
;427/385.5,393.5,407.1,388.1,388.3,409 ;524/512 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Schofer; Joseph L.
Assistant Examiner: Delmendo; R. H.
Attorney, Agent or Firm: Werner; Frank G. Marshall; Paul
L.
Claims
We claim:
1. A process for coating a substrate using a pigmented basecoat
composition and a clear aqueous topcoat composition, said clear
aqueous topcoat composition having a solids content of less than
about 55% by weight and comprising a water-thinnable polyacrylate
resin as binder and an aminoplast resin as crosslinking agent, said
process comprising the steps of:
(1) applying a pigmented basecoat composition to the substrate
surface, the applied basecoat composition forming a polymeric
film,
(2) applying a clear aqueous topcoat composition to the basecoat,
and subsequently
(3) baking the basecoat and the topcoat together,
wherein said water-thinnable polyacrylate resin is obtained
(I) by adding to an organic solvent or mixture of organic solvents,
in the presence of at least one polymerization initiator, a mixture
of
(a1) 2.5-15% by weight of an ethylenically unsaturated monomer
which contains at least one carboxyl group per molecule and is
copolymerizable with (b1), (b2) and (b3), or a mixture of such
monomers (a1), together with
(a2) 0-6% by weight of an ethylenically unsaturated monomer which
is free from carboxyl groups and is copolymerizable with (b1),
(b2), (b3) and (a1), or a mixture of such monomers (a2),
either successively or alternately in part amounts with a mixture
(b) consisting of
(b1) 40-87.5% by weight of a (meth)acrylic ester which is
copolymerizable with (b2), (b3), (a1) and (a2) and is essentially
free from carboxyl groups, or a mixture of such (meth)acrylic
esters, and
(b2) 10-45% by weight of an ethylenically unsaturated monomer which
is copolymerizable with (b1), (b3), (a1) and (a2), contains at
least one hydroxyl group per molecule and is essentially free from
carboxyl groups, or a mixture of such monomers (b2), and,
(b3) 0-25% by weight of an ethylenically unsaturated monomer,
different from (a2), which is copolymerizable with (b1), (b2), (a1)
and (a2) and is essentially free from carboxyl groups, or a mixture
of such monomers (b3), and
(II) by at least partly neutralizing the resultant polyacrylate
resin after the termination of the polymerization and dispersing it
in water, the total of the weight proportions of (a1), (a2), (b1),
(b2) and (b3) being always 100% by weight, and the amount of (b1),
(b2), (b3), (a1) and (a2) being chosen such that the polyacrylate
resin has a hydroxyl value of 40 to 200, an acid value of 20 to
100, and a glass transition temperature (T.sub.g) of -40.degree. C.
to +60.degree. C.
2. The process as claimed in claim 1, wherein the basecoat
composition applied in step 1 is an aqueous basecoat
composition.
3. The process as claimed in claim 1, wherein no component (a2) is
used.
4. The process as claimed in claim 1, wherein no component (a2) is
used and wherein an amount of the component (b) (amount 1)
consisting of 30 to 70% by weight of the total amount of the
component (b) to be used is added first, after which an amount of
the component (a1) (amount 2) consisting of 30 to 70% by weight of
the total amount of the component (a1) to be used is added, after
which an amount of the component (b) (amount 3) consisting of 30 to
70% by weight of the total amount of the component (b) to be used
is added and finally an amount of component (a1) (amount 4)
consisting of 30 to 70% by weight of the total amount of the
component (a1) to be used is added, the total of the weight
proportions of amounts 1 and 3 and the total of the weight
proportions of amounts 2 and 4 always being 100% by weight.
5. The process as claimed in claim 1, wherein no component (a2) is
used and wherein an amount of the component (b) (amount 1)
consisting of 30 to 70% by weight of the total amount of the
component (b) to be used is added first, after which an amount of
the component (a1) (amount 2) consisting of 30 to 70% by weight of
the total amount of the component (a1) to be used is added, after
which an amount of the component (b) (amount 3) consisting of 30 to
70% by weight of the total amount of the component (b) to be used
is added and finally an amount of component (a1) (amount 4)
consisting of 30 to 70% by weight of the total amount of the
component (a1) to be used is added, the total of the weight
proportions of amounts 1 and 3 and the total of the weight
proportions of amounts 2 and 4 always being 100% by weight, and
wherein polymerization takes place at a temperature of 80.degree.
to 160.degree. C. in the presence of at least one free-radical
initiator, and the addition of the amount 1 lasts 1 to 4 hours, the
addition of amounts 2 lasts 5 to 30 minutes, the addition of the
amount 3 lasts 1 to 4 hours and the addition of the amount 4 lasts
5 to 30 minutes.
Description
FIELD OF THE INVENTION
The invention relates to a process for the preparation of a
multicoat protective and/or decorative coating on a substrate
surface, in which process
(1) a suitable pigmented basecoat composition is applied to the
substrate surface,
(2) the composition applied in stage (1) forms a polymeric
film,
(3) a clear aqueous topcoat composition comprising a
water-thinnable polyacrylate resin as binder and an aminoplast
resin as crosslinking agent is applied to the basecoat obtained in
this manner, and subsequently
(4) the basecoat and the topcoat are baked together.
The invention also relates to aqueous coating compositions,
water-thinnable polyacrylate resins and a process for the
preparation of water-thinnable polyacrylate resins.
BACKGROUND ART
The process in accordance with the preamble of patent claim 1 is
known. It is used especially for the production of automotive
metallic finishes (cf. for example U.S. Pat. No. 3,639,147,
DE-A-3,333,072 and EP-A-38,127).
On economic and ecological grounds it is desirable to employ
aqueous topcoat compositions in stage (3) of the basecoat-clearcoat
process described above.
The topcoat compositions must be sprayable with the aid of
automatic painting equipment. For this purpose their solids content
must be high enough at spraying viscosity to furnish paint films of
adequate film thickness with one to two cross-passes of the
spray-gun and they must furnish baked paint films of good
appearance (good flow-out, high gloss, good topcoat condition . . .
).
The aqueous topcoat composition disclosed in Example 2 of
EP-A-38,127 does not meet all of the above requirements.
The object forming the basis of the present invention consists in
the preparation of aqueous topcoat compositions which meet the
above requirements.
SUMMARY OF THE INVENTION
Surprisingly, this object is achieved using topcoat compositions
which comprise a water-thinnable polyacrylate resin as binder,
which resin can be obtained
(I) by adding to an organic solvent or mixture of organic solvents,
in the presence of at least one polymerization initiator, a mixture
of
(a1) 2.5-15%, preferably 3-7%, by weight of an ethylenically
unsaturated monomer which contains at least one carboxyl group per
molecule and is copolymerizable with (b1), (b2) and (b3), or a
mixture of such monomers, together with
(a2) 0-6% by weight of an ethylenically unsaturated monomer which
is free from carboxyl groups and is copolymerizable with (b1),
(b2), (b3) and (a1), or a mixture of such monomers,
either successively or alternately in part amounts with a mixture
(b) consisting of
(b1) 40-87.5%, preferably 55-80%, by weight of a (meth)acrylic
ester which is copolymerizable with (b2), (b3), (a1) and (a2) and
is essentially free from carboxyl groups, or a mixture of such
(meth)acrylic esters, and
(b2) 10-45%, preferably 12-35%, by weight of an ethylenically
unsaturated monomer which is copolymerizable with (b1), (b3), (a1)
and (a2), contains at least one hydroxyl group per molecule and is
essentially free from carboxyl groups, or a mixture of such
monomers, and
(b3) 0-25%, preferably 8-18%, by weight of an ethylenically
unsaturated monomer, different from (a2), which is copolymerizable
with (b1), (b2), (a1) and (a2) and is essentially free from
carboxyl groups, or a mixture of such monomers, and
(II) by at least partly neutralizing the resultant polyacrylate
resin after the termination of the polymerization and dispersing it
in water, the total of the weight proportions of (a1), (a2), (b1),
(b2), and (b3) being always 100% by weight and the type and amount
of (b1), (b2), (b3), (a1) and (a2) being chosen such that the
polyacrylate resin has a hydroxyl value of 40 to 200, preferably 60
to 140, an acid value of 20 to 100, preferably 25 to 50, and a
glass transition temperature (T.sub.G) of -40.degree. C. to
+60.degree. C., preferably -20.degree. C. to +40.degree. C.
BRIEF DESCRIPTION OF THE DRAWING
The accompanying FIGURE is a plot of viscosity as a function of
solids content for several of the compositions prepared in the
Examples herein.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The pigmented basecoat compositions to be applied in stage (1) of
the process according to the invention are well known (cf. for
example U.S. Pat. No. 3,639,147, EP-A-38,127, DE-A-3,333,072 and
EP-A-279,813). The basecoat compositions used in stage (1)
preferably comprise metal pigments, especially aluminum pigments,
metallic finishes being obtained in this manner.
Before the preparation of the polyacrylate resins to be used
according to the invention is described in detail, two terms will
be clarified first:
1) in a number of places "(meth)acrylic acid" is used as an
abbreviation for "methacrylic or acrylic acid".
2) The term "essentially free from carboxyl groups" is intended to
indicate that the components (b1), (b2) and (b3) may possess a low
content of carboxyl groups (not more, however, than would impart to
a polyacrylate resin prepared from the components (b1), (b2) and
(b3) an acid value greater than 10). It is preferred, however, that
the content of carboxyl groups of the components (b1), (b2) and
(b3) is kept as low as possible. The use of components (b1), (b2)
and (b3) which are free from carboxyl groups is particularly
preferred.
Any ethylenically unsaturated monomer which contains at least one
carboxyl group per molecule and is copolymerizable with (b1), (b2)
and (b3), or a mixture of such monomers, may be employed as
component (al) for the preparation of the polyacrylate resins to be
used according to the invention. Acrylic and/or methacrylic acid
are preferably used as the component (a1). However, other
ethylenically unsaturated acids having up to 6 carbon atoms in the
molecule may be used. Examples of such acids are ethacrylic acid,
crotonic acid, maleic acid, fumaric acid and itaconic acid.
Mono(meth)acryloylhydroxyethyl succinate and
mono(meth)acryloylhydroxyethyl phthalate, for example, may also be
used as the component (a1).
Any ethylenically unsaturated monomer which is copolymerizable with
(b1), (b2), (b3) and (a1), or a mixture of such monomers, may be
used as the component (a2). All the monomers listed in the
description of the components (b1), (b2) and (b3) may be used as
the component (a2).
It is preferred to employ 100% by weight of the component (a1). In
other words: It is preferred to use the component (a1) without
admixture of the component (a2).
Any ester of (meth)acrylic acid which is copolymerizable with (b2),
(b3), (a1) and (a2) and is essentially free from carboxyl groups,
or a mixture of such (meth)acrylates, may be used as component
(b1). Examples of these are alkyl acrylates and alkyl methacrylates
having up to 20 carbon atoms in the alkyl radical, for example
methyl, ethyl, propyl, butyl, hexyl, ethylhexyl, stearyl and lauryl
acrylate and methacrylate. Preferably used as the component (b1)
are mixtures of alkyl acrylates and/or alkyl methacrylates which
comprise at least 25% by weight of n-butyl or t-butyl acrylate
and/or n-butyl or t-butyl methacrylate.
Any ethylenically unsaturated monomer which is copolymerizable with
(b1), (b3), (a1) and (a2), contains at least one hydroxyl group per
molecule and is essentially free from carboxyl groups, or a mixture
of such monomers, may be used as the component (b2). Examples are
hydroxyalkyl esters of acrylic acid, methacrylic acid or another
.alpha., .beta.-ethylenically unsaturated carboxylic acid. These
esters may be derived from an alkylene glycol which is esterified
with the acid, or they may be obtained by reacting the acid with an
alkylene oxide. Hydroxyalkyl esters of acrylic acid and methacrylic
acid in which the hydroxyalkyl group contains up to 4 carbon atoms,
reaction products of cyclic esters, for example
.epsilon.-caprolactone and these hydroxyalkyl esters or mixtures of
these hydroxyalkyl esters and .epsilon.-caprolactone-modified
hydroxyalkyl esters are preferably employed as the component (b2).
Examples of such hydroxyalkyl esters are 2-hydroxyethyl acrylate,
2-hydroxypropyl acrylate, 3-hydroxypropyl acrylate, 2-hydroxypropyl
methacrylate, 3-hydroxypropyl methacrylate, 2-hydroxyethyl
methacrylate, 4-hydroxybutyl acrylate and 4-hydroxybutyl
methacrylate. Corresponding esters of other unsaturated acids, for
example ethacrylic acid, crotonic acid and similar acids having up
to about 6 carbon atoms per molecule, may be also used.
Any ethylenically unsaturated monomer which is copolymerizable with
(b1), (b2), (a1) and (a2) and is essentially free from carboxyl
groups, or a mixture of such monomers, may be employed as the
component (b3). Vinylaromatic hydrocarbons, such as styrene,
.alpha.-alkylstyrene and vinyltoluene, are preferably used as the
component (b3).
The polyacrylate resins employed according to the invention may be
prepared by adding 60 to 100% by weight of the component (a1)
together with 0-40% by weight of the component (a2) to an organic
solvent or mixture of solvents and polymerizing the mixture in the
presence of at least one polymerization initiator and, after
addition of the components (a1) and (a2) has been completed, adding
a mixture (b), consisting of the components (b1) and (b2), with or
without (b3) to the organic solvent or mixture of solvents and
polymerizing the mixture there in the presence of at least one
polymerization initiator. After the termination of the
polymerization the resultant polyacrylate resin is at least
partially neutralized and dispersed in water. The total of the
weight proportions of (a1) and (a2) is always 100% by weight. The
type and amount of the components (a1), (a2), (b1), (b2) and (b3)
are chosen such that the polyacrylate resin has a hydroxyl value of
40 to 200, preferably 60 to 140, an acid value of 20 to 100,
preferably 25 to 50, and a glass transition temperature (T.sub.G)
of -40.degree. C. to +60.degree. C., preferably -20.degree. C. to
+40.degree. C.
The component (a1) is added to the organic solvent or mixture of
solvents--with or without the component (a2)--preferably within 10
to 60 minutes, particularly preferably within 15 to 45 minutes, and
is polymerized in solution in the presence of at least one
free-radical initiator. The mixture (b) is added to the organic
solvent or mixture of solvents preferably within 2 to 8 hours,
particularly preferably within 3 to 6 hours, and is polymerized in
solution in the presence of at least one free-radical
initiator.
The polyacrylate resins to be used according to the invention may
also be prepared by adding the component (a1) (with or without
(a2)) and the mixture (b) to an organic solvent or mixture of
solvents in part amounts alternately and polymerizing the mixture
in solution in the presence of at least one free-radical initiator.
The part amounts should in each case consist of at least 10% by
weight of the total amount of the components (a1) and (a2) and the
mixture (b) to be employed. After the termination of the
polymerization the resultant polyacrylate resin is at least partly
neutralized and dispersed in water. The total amount of weight
proportions of (a1) and (a2) is always 100% by weight. The type and
amount of the components (a1), (a2), (b1), (b2) and (b3) are chosen
such that the polyacrylate resin has a hydroxyl value of 40 to 200,
preferably 60 to 140, an acid value of 20 to 100, preferably 25 to
50, and a glass transition temperature (T.sub.G) of -40.degree. C.
to +60.degree. C., preferably -20.degree. C. to +40.degree. C.
In a preferred embodiment of this method of preparation, in a first
stage an amount of the component (b) (amount 1) consisting of 30 to
70% by weight, preferably 40 to 60% by weight, particularly
preferably 50% by weight, of the total amount of the component (b)
to be used is added to the organic solvent or mixture of solvents
and is polymerized in the presence of at least one free-radical
initiator. When the addition of the amount 1 is completed, in a
second stage an amount of the component (a1) (amount 2) consisting
of 30 to 70% by weight, preferably 40 to 60% by weight,
particularly preferably 50% by weight, of the total amount of the
component (a1) to be used is added to the organic solvent or
mixture of solvents and is polymerized in the presence of at least
one free-radical initiator. When the addition of the amount 2 is
completed, in a third stage an amount of the component (b) (amount
3) consisting of 30 to 70% by weight, preferably 40 to 60% by
weight, particularly preferably 50% by weight, of the total amount
of the component (b) to be used, is added to the organic solvent or
mixture of solvents and is polymerized in the presence of at least
one free radical initiator. When the addition of the amount 3 is
completed, in the fourth stage an amount of the component (a1)
(amount 4) consisting of 30 to 70% by weight, preferably 40 to 60%
by weight, particularly preferably 50% by weight, of the total
amount of the component (a1) to be used is added to the organic
solvent or mixture of solvents and is polymerized in the presence
of at least one free radical initiator.
After the termination of the polymerization the resultant
polyacrylate resin is at least partly neutralized and dispersed in
water. The total of the weight proportions of (a1) and (a2) is
always 100% by weight. The type and amount of components (a1),
(a2), (b1), (b2) and (b3) are chosen such that the polyacrylate
resin has a hydroxyl value of 40 to 200, preferably 60 to 140, an
acid value of 20 to 100, preferably 25 to 50, and a glass
transition temperature (T.sub.G) of -40.degree. C. to +60.degree.
C., preferably -20.degree. C. to +40.degree. C.
The addition of the amounts 1 and 3 is preferably carried out
within 1 to 4 hours, particularly preferably within 1.5 to 3 hours.
The addition of the amounts 2 and 4 is preferably carried out
within 5 to 30 minutes, particularly preferably within 7 to 20
minutes.
The solvents and polymerization initiators which are customary for
the preparation of polyacrylate resins and are suitable for the
preparation of aqueous dispersions are employed as the organic
solvents and polymerization initiators. Examples of suitable
solvents are butyl glycol, 2-methoxypropanol, n-butanol,
methoxybutanol, n-propanol, ethylene glycol monomethyl ether,
ethylene glycol monoethyl ether, ethylene glycol monobutyl ether,
diethylene glycol monomethyl ether, diethylene glycol monoethyl
ether, diethylene glycol diethyl ether, diethylene glycol monobutyl
ether and 3-methyl-3-methoxybutanol. Examples of suitable
polymerization initiators are free-radical initiators, for example
benzoyl peroxide, azobisisobutyronitrile and t-butyl perbenzoate.
The polymerization is performed at a temperature of 80.degree. to
160.degree. C., preferably 120.degree. to 160.degree. C. The
polymerization is completed when all the monomers used have
essentially completely reacted.
The type and amount of the components (a1), (a2), (b1), (b2) and
(b3) are chosen such that the reaction product has a hydroxyl value
of 40 to 200, preferably 60 to 140, an acid value of 20 to 100,
preferably 25 to 50 and a glass transition temperature (T.sub.G) of
-40.degree. C. to +60.degree. C., preferably -20.degree. C. to
+40.degree. C.
The approximate glass transition temperatures of polyacrylate
resins may be calculated using the following formula: ##EQU1##
T.sub.G =glass transition temperature of the polyacrylate resin
x=number of the various monomers incorporated in the polyacrylate
resin
W.sub.n =weight proportion of the nth monomer
T.sub.Gn =glass transition temperature of the homopolymer obtained
from the nth monomer
The amount and the rate of addition of the initiator is preferably
controlled in such a way that the resultant polyacrylate resin has
a number-average molecular weight of 2,500 to 20,000. It is
particularly preferred to commence the addition of the initiator at
the same time as the addition of the polymerizable components (a1),
(a2) and (b) and to terminate it about half an hour after
completing the addition of the polymerizable components (a1), (a2)
and (b). The reaction mixture is subsequently kept at the
polymerization temperature for long enough (as a rule about 1.5
hours) for all the used monomers to have essentially completely
reacted. The expression "essentially completely reacted" is
intended to signify that preferably 100% by weight of the monomers
used have reacted, but that there is also the possibility that a
small residual content of monomers, not greater than 0.5% by weight
based on the weight of the reaction mixture, may remain
unreacted.
The polyacrylate resins according to the invention are prepared by
using 40 to 87.5% by weight, preferably 55 to 80% by weight, of the
component (b1), 10 to 45% by weight, preferably 12 to 35% by
weight, of the component (b2), 0 to 25% by weight, preferably 8 to
18% by weight, of the component (b3), 2.5 to 15% by weight,
preferably 3 to 7% by weight, of the component (a1) and 0 to 6% by
weight of the component (a2), the total of the weight proportions
of (b1), (b2), (b3), (a1) and (a2) being always 100% by weight.
After the termination of the polymerization the resultant
polyacrylate resin is at least partly neutralized and dispersed in
water.
Organic bases as well as inorganic bases, such as ammonia and
hydrazine, may be used for the neutralization. Primary, secondary
and tertiary amines, for example ethylamine, propylamine,
dimethylamine, dibutylamine, cyclohexylamine, benzylamine,
morpholine, piperidine and triethanolamine are preferably employed.
Tertiary amines, especially dimethylethanolamine, triethylamine,
tripropylamine and tributylamine, are particularly preferably used
as neutralization agents.
The neutralization reaction is generally performed by mixing the
neutralizing base with the polyacrylate resin, in which case enough
base is preferably used for the topcoat coating composition to have
a pH of 7-8.5, preferably 7.2 to 7.8.
The partly or completely neutralized polyacrylate resin is
subsequently dispersed by the addition of water, an aqueous
polyacrylate resin dispersion being formed. A part or all of the
organic solvent may be distilled off, if desired. The polyacrylate
resin dispersions according to the invention contain polyacrylate
resin particles whose average particle size is preferably between
60 and 250 nm (method of measurement: laser light scatter;
measuring instrument: Malvern Autosizer 2C).
Topcoat compositions according to the invention are produced from
the aqueous polyacrylate resin dispersions obtained in this manner
by generally known methods by the admixture of an aminoplast resin
and, if appropriate, of other additives, for example flow control
agents, UV stabilizers, transparent pigments etc.
The topcoat compositions according to the invention preferably
contain enough aminoplast resin for the weight ratio of the
polyacrylate resin solids to the aminoplast resin solids to be
60:40 to 90:10, particularly preferably 70:30 to 85:15.
In principle any aminoplast resin which can be processed with the
polyacrylate resin according to the invention to form a stable
topcoat coating composition may be used as crosslinking agent.
Melamine-formaldehyde resins which are partly or completely
etherified with aliphatic alcohols having preferably 1 to 4 carbon
atoms per molecule are preferred as crosslinking agents.
Prior to being applied, the topcoat coating compositions according
to the invention are adjusted to spraying viscosity (generally 20
to 40 seconds efflux time from a DIN Cup 4 (DIN 53211 (1974))) and
a pH of 7.0 to 8.5, preferably 7.2 to 7.8.
The topcoat compositions according to the invention
have at spraying viscosity a solids content (20 to 45% by weight,
preferably 32 to 40% by weight) which is high enough to furnish
paint films of adequate film thickness with one or two cross-passes
of the spray-gun (the thickness of the baked paint film should
preferably be between 25 and 45 .mu.m), and
furnish baked paint films of very good appearance (good flow-out,
high gloss, good topcoat conditions . . . ) and good mechanical
application properties, and
comprise a relatively low proportion of organic co-solvents (less
than 35% by weight based on the total solids contents of binders
and crosslinking agents).
If the topcoat compositions according to the invention are employed
in conjunction with water-thinnable basecoat compositions for the
production of metallic finishes, then metallic finishes are
obtained in which the clear topcoat adheres particularly well to
the basecoat.
The topcoat compositions according to the invention may comprise in
addition to the polyacrylate resin used according to the invention
also crosslinked polymer microparticles such as those disclosed,
for example, in EP-A-38,127, and/or further compatible resins, for
example water-thinnable or water-soluble polyacrylate resins,
polyester resins, alkyd resins or epoxy resin esters, and they may
also be pigmented.
The invention is explained in greater detail in the Examples
below:
A. Preparation of water-thinnable polyacrylate resins according to
the invention
TABLE 1 ______________________________________ Amounts added in
parts by weight Component Monomer P1 P2 P3
______________________________________ (a1) Acrylic acid 5.0 5.0
5.0 (b1) Butyl acrylate 22.0 22.0 22.0 Butyl methacrylate 20.0 20.0
20.0 Methyl methacrylate 15.0 15.0 15.0 (b2) 2-Hydroxypropyl 23.0
23.0 23.0 acrylate (b3) Styrene 15.0 15.0 15.0
______________________________________
P1> 20 parts by weight of butyl glycol are introduced into a 4
liter steel reaction vessel provided with two monomer inlets, an
initiator inlet, a stirrer, a thermometer, an oil bath and a reflux
condenser and are heated to 140.degree. C. A solution of 4.5 parts
by weight of butyl perbenzoate in 5 parts by weight of butyl glycol
is then added at such a rate that the addition is concluded in 4
hours 30 minutes. At the same time as the addition of the butyl
perbenzoate solution commences, the addition of the component (a1)
commences as well (cf. Table 1). The component (a1) is added at
such a rate that the addition is concluded in 20 minutes. After all
of the component (a1) has been added, the mixture of (b1), (b2) and
(b3) is added within 3 hours 40 minutes. The reaction mixture is
subsequently kept at 140.degree. C. for long enough for the solids
content of the resin solution to be at least 80% by weight (1 hour,
130.degree. C.). The resin solution obtained in this manner is
neutralized with dimethylethanolamine at 95.degree. C. until a
degree of neutralization of 80% is reached. Subsequently enough
water is added for the solids content of the dispersion to be about
60% by weight.
P2> 20 parts by weight of butyl glycol are introduced into a 4
liter steel reaction vessel provided with two monomer inlets, an
initiator inlet, a stirrer, a thermometer, an oil bath and a reflux
condenser and are heated to 140.degree. C. A solution of 4.5 parts
by weight of butyl perbenzoate in 5 parts by weight of butyl glycol
is then added at such a rate that the addition is concluded in 4
hours 30 minutes.
At the same time as the addition of the butyl perbenzoate solution
commences, the addition of 50% by weight of the total amount of the
mixture of (b1), (b2) and (b3) to be used commences as well. This
amount of the mixture of (b1), (b2) and (b3) is added at such a
rate that the addition is concluded after 1 hour 53 minutes. When
the addition of the first part of the mixture of (b1), (b2) and
(b3) is concluded, 50% by weight of the total amount of the
component (a1) to be used is added within 7 minutes. The remaining
50% by weight of the total amount of the mixture of (b1), (b2) and
(b3) to be used is added subsequently within 1 hour 52 minutes.
After all of the remaining 50% by weight of the total amount of the
mixture (b1), (b2) and (b3) to be used has been added, the
remaining 50% by weight of the total amount of the component (a1)
to be used is added within 8 minutes. The reaction mixture is
subsequently kept at 140.degree. C. for long enough for the solids
content of the resin solution to be at least 80% by weight (1 hour,
130.degree. C.).
The resin solution obtained in this manner is neutralized with
dimethylethanolamine at 95.degree. C. until a degree of
neutralization of 80% is reached. Subsequently enough water is
added for the solids content of the dispersion to be about 60% by
weight.
P3> 20 parts by weight of butyl glycol are introduced into a 4
liter steel reaction vessel provided with two monomer inlets, an
initiator inlet, a stirrer, a thermometer, an oil bath and a reflux
condenser and are heated to 140.degree. C. A solution of 4.5 parts
by weight of butyl perbenzoate in 5 parts by weight of butyl glycol
is then added at such a rate that the addition is concluded in 4
hours 30 minutes. At the same time as the addition of the butyl
perbenzoate solution commences, the addition of 33.3% by weight of
the total amount of the component (a1) to be used (amount 1)
commences as well. The amount 1 is added at such a rate that the
addition is concluded in 10 minutes. After the addition of the
amount 1 has been concluded, 50% by weight of the total amount of
the mixture of (b1), (b2) and (b3) to be used (amount 2) is added
within 1 hour 55 minutes. After all of the amount 2 has been added,
33.3% by weight of the total amount of the component (a1) to be
used (amount 3) is added within 10 minutes. When the addition of
the amount 3 is concluded, the remaining 50% by weight of the total
amount of the mixture (b1), (b2) and (b3) to be used (amount 4) is
added within 1 hour 55 minutes. After all of the amount 4 has been
added, the remaining 33.33% by weight of the total amount of the
component (a1) to be used (amount 5) is finally added within 10
minutes. The reaction mixture is subsequently kept at 140.degree.
C. for long enough for the solids content of the resin solution to
be at least 80% by weight (1 hour, 130.degree. C.).
The resin solution obtained in this manner is neutralized with
dimethylethanolamine at 95.degree. C. until a degree of
neutralization of 80% is reached. Subsequently enough water is
added for the solids content of the dispersion to be about 60% by
weight.
The resultant dispersions have the following characteristics:
TABLE 2 ______________________________________ P1 P2 P3
______________________________________ Solids (% by weight; 1 h,
130.degree. C.) 59.6 59.0 59.6 Acid value 29.9 38.9 33.0 Degree of
neutralization (%) 80 80 80 OH value 100 100 100 Particle size 1)
(nm) 119 190 67 ______________________________________
1) The particle size is determined by laser light scatter.
Apparatus: Malvern Autosizer 2C.
B. Preparation of clear aqueous topcoat composition according to
the invention
Clear topcoat compositions are prepared from 60.00% by weight of a
polyacrylate dispersion prepared in section A, 29.95% by weight of
distilled water, 10.00% by weight of a 90% strength solution of a
commercial water-thinnable melamine-formaldehyde resin in
isobutanol (Cymel 327) and 0.05% by weight of a commercial flow
control agent (Fluorad FC 430); they are adjusted to spraying
viscosity (24 seconds efflux time from DIN Cup 4 (DIN 53211
(1974))) and a pH of 7.4 to 7.5 using 10% aqueous
dimethylethanolamine solution and distilled water. The topcoat
compositions comprise 20% by weight of organic co-solvents based on
total solids content of polyacrylate resin and
melamine-formaldehyde resin.
C. Application of the clear topcoat compositions according to the
invention and testing of the baked paint films
A water-thinnable basecoat composition pigmented with aluminum
platelets in accordance with EP-A-279,813 is applied to a
phosphated steel sheet coated with a commercial electrocoating
paint and a commercial filler in such a way that the dry film
thickness is 12-15 .mu.m. The applied basecoat composition is dried
for 10 minutes at room temperature and for 10 minutes at 80.degree.
C.; it is then sprayed with the topcoat composition from section B
in two cross-passes of the spray-gun with one minute flash-off
period between the passes. The paint is finally dried for 20
minutes at room temperature and baked for 30 minutes at 130.degree.
C. in a circulating-air oven. The multicoat coatings obtained in
this manner were subjected to a number of tests. The results of
these are summarized in the following Table:
TABLE 3 ______________________________________ P1 P2 P3
______________________________________ Application solids content
41.0 35.0 36.0 (1 h, 130.degree. C., % by weight) Thickness of the
clear topcoat 31 30 30 (.mu.m) Crosshatch test (1) 0-1 0-1 0 Gloss
(2) 86 86 86 ______________________________________ (1) Test in
accordance with DIN 53151 including the Tesa pulloff test (2)
Degree of gloss in accordance with DIN 67530, angle 20
B. Comparison example
The monomers (a1), (b1), (b2) and (b3) used for the preparation of
the polyacrylate resins P1 and P3 according to the invention are
mixed and polymerized. The polymerization conditions are identical
with those prevailing in the preparation of the polyacrylate resins
P1 and P3 according to the invention. The sole difference from the
preparation of the polyacrylate resins according to the invention
is that the component (a1) is not added prior to the mixing of
(b1), (b2) and (b3) as in the case of P1 and not alternately as in
the case of P3, but together with (b1), (b2) and (b3). An aqueous
topcoat composition prepared from the polyacrylate resin V thus
prepared in accordance with B has a solids content as low as 24.1%
by weight at a viscosity of 24 DIN seconds (efflux time from a DIN
Cup 4 (DIN 53211 (1974))) and a pH of 7.4 to 7.5.
The diagram shown in the accompanying FIGURE demonstrates the
viscosities of the polyacrylate dispersions P1 and P3 prepared in
accordance with A and thinned with water as a function of the
solids content of the dispersions compared with a similar
dispersion of the polyacrylate resin V (pH of the dispersions:
7.8). Curves 1, 2, and 3 represent the values for V, P3, and P1,
respectively. It becomes clear that the viscosities of the
polyacrylate resin dispersions P1 and P3 according to the invention
are, at solids contents of less than about 55% by weight, are lower
than the viscosity of the dispersion of the polyacrylate resin
V.
* * * * *